Heterogeneously deployed networks with network nodes of different nominal transmit powers and (at least partially) overlapping coverage areas are considered to be an interesting future deployment strategy for cellular networks. An introduction to such network deployments can be found in S. Parkvall et al., “Heterogeneous network deployments in LTE”, Ericsson Review, No. 2, 2011, where LTE stands for the Long Term Evolution standard of the 3rd Generation Partnership Project (3GPP).
FIG. 1 illustrates an example of a heterogeneously deployed network 10 with a low-power network node 12 and a high-power network node 14. The low-power network node 12 (in the following also called “pico node”) is typically assumed to offer high data rates (Mbit/s) and high capacity (users/m2 or Mbit/s/m2) in local areas where this is needed or desired. On the other hand, the high-power network node 14 (in the following also called “macro node”) is assumed to provide large-area coverage.
With reference to FIG. 2, in practice the macro node 14 may correspond to an existing cell 16 (a “macro cell”), while the pico node 12 may be deployed later to locally extend at least one of the capacity and achievable data rate within the coverage area of the macro cell 16 (where needed). In the scenario illustrated in FIG. 2, the pico node 12 corresponds to a cell 18 of its own (a “pico cell”). This means that, in addition to downlink and uplink data transmission or reception, the pico node 12 also transmits the full set of common signals and channels typically associated with a cell. For this reason the pico node 12 can be detected and selected (i.e., connected to) by a terminal device 20 within the pico cell 18.
In the exemplary LTE context illustrated in FIG. 2, the signals and channels transmitted by the pico node 12 for the terminal device 20 connected to the pico cell 18 include:                The Primary and Secondary Synchronization Signals (PSS and SSS), corresponding to the Physical Cell Identity of the pico cell 18.        The Cell-specific Reference Signals (CRS), also corresponding to the Physical Cell Identity of the pico cell 18. The CRS may, for example, be used for downlink channel estimation to enable coherent demodulation of downlink transmissions by the terminal device 20.        The Physical Broadcast Channel (PBCH), with corresponding pico-cell system information (additional system information may be transmitted on the Physical Downlink Shared Channel, PDSCH).        
As the pico node 12 illustrated in FIG. 2 corresponds to a cell 18 of its own, also so-called Layer 1 (L1) and Layer 2 (L2) control signaling on the Physical Downlink Control Channel (PDCCH) (as well as on the Physical Control Format Indicator Channel, PCFICH, and the Physical Hybrid-ARQ Indicator Channel, PHICH) are transmitted from the pico node 12 to the connected terminal device 20. Such L1/L2 control signaling is performed in addition to downlink data transmission on the PDSCH and provides, for example, downlink and uplink scheduling information and Hybrid-ARQ-related information to the terminal device 20 within the pico cell 18.
As an alternative to the deployment scenario illustrated in FIG. 2, the pico node 12 within the heterogeneous network deployment may not correspond to a cell of its own but may just provide a data-rate and capacity “extension” of the macro cell 16 as shown in FIG. 3. Such a deployment is sometimes also referred to as “soft cell” (or “shared cell”).
In a soft cell deployment, at least the CRS, PBCH, PSS and SSS are transmitted from the macro node 14. The PDSCH can be transmitted from the pico node 12. To allow for demodulation and detection of the PDSCH, despite the fact that no CRS is transmitted from the pico node 12, so-called Demodulation Reference Signals (DM-RSs) may be transmitted from the pico node 12 together with the PDSCH. The DM-RSs, which are terminal-specific, can then be used by the terminal device 20 for PDSCH demodulation and detection as known in the art.
Successfully receiving data from the pico node 12 that does not transmit CRS as described above requires DM-RS support in the terminal device 20 (“non-legacy” terminal). In LTE, DM-RS-based PDSCH reception is supported in Rel-10 and for Frequency Division Duplex (FDD), while for the L1/L2 control signalling, DM-RS-based reception is planned for Rel-11.
For terminal devices not supporting DM-RS-based reception (“legacy” terminals) one possibility for a soft cell scenario is a Single Frequency Network (SFN)-type of operation as illustrated in FIG. 4. In essence, identical copies of the signals and channels required by a legacy terminal are transmitted simultaneously from the macro node 14 and the pico node 12 during SFN operation. From a terminal perspective this looks like a single transmission. SFN operation will generally only provide a gain of the Signal to Interference-plus-Noise Ratio (SINR), which can be translated into a higher data rate, but will typically not result in a capacity improvement as transmission resources are not straight forward to re-use across sites within the same cell.
Summarizing the above, using the shared cell approach illustrated in FIG. 3 with DM-RSs to transmit data from the pico node 12 to the terminal device 20 provides gains in both capacity and data rates for non-legacy terminals supporting DM-RSs. It also provides benefits in energy efficiency as the pico node 12 needs to be active only at those points in time when it is involved in data transmissions to the terminal device 20. However, legacy terminals not supporting DM-RSs cannot benefit from the pico node 12 in this case. Alternatively, SFN operation between the pico node 12 and the macro node 14 may allow a legacy terminal to benefit from provision of the pico node 12, but in this case many of the benefits possible to obtain with non-legacy terminals, such as capacity and energy efficiency, cannot easily be achieved.